Polymer electrolyte membranes based on polybenzimidazole (PBI) are known for use in high-temperature fuel cells (U.S. Pat. No. 5,525,436, WO 200118894 A2, DE 103 01 810 A1, DE 101 55 543 C2 and M. Rikukawa, K. Sanui, Prog. Polym. Sci. 2000, 1463-1502). After being doped with phosphoric acid, the basic polymer forms a proton-conducting phase, in which proton transport does not depend on the presence of water, thus permitting operation of these fuel cells in the temperature range up to 200° C. The doping process does not impair the high thermal stability of PBI, although it does reduce the mechanical stability. It is therefore common practice to cross-link the polybenzimidazole by covalent bonds. As cross-linking reagents there are used bifunctional hydrocarbon-based compounds containing epoxide or isocyanate groups (WO 200044816 A1, U.S. Pat. No. 6,790,553 B1, DE 103 01 810 A), which cross-link the polymer chains with one another by means of reaction with the NH groups of the polybenzimidazole. However, the chemical and thermal stability of the cross-linking points, especially the stability of the cross-linking organic groups to oxidizing agents at higher temperatures, is limited. Such aliphatic cross-linking chains do not contribute in any way to proton conductivity. Because of restricted mobility of the overall system and also because of limited ability of the cross-linked polymer to absorb the doping agent that imparts proton conductivity, the conductivity of the membranes can be additionally restricted, if PBI is cross-linked via diepoxides or diisocyanates.
The stability of an organic polymer matrix can be improved by incorporation of silicate reinforcing material. From the prior art there are known, for example, hybrid membranes, composed respectively of hydroxypropylcellulose (HPC), polyvinyl alcohol (PVA) and polyvinylidene fluoride (PVDF) and silicate material. These hybrid membranes are produced by mixing the polymer with alkoxy-substituted silanes, such as tetraethoxysilane (TEOS), the inorganic material being deposited in the organic polymer matrix by means of an acid-catalyzed sol-gel process. The interaction between organic and inorganic phase can be mediated by hydrogen-bridge bonds, and the mechanical stability of these hybrid materials increases with increasing content of silicate material (S. Yano, Materials Science and Engineering C6 (1998), 75-90). For the inorganic phase, however, the attainable degree of condensation by acid-catalyzed condensation is only 65 to 75%, whereas degrees of condensation of 80 to 90% are attained under base catalysis (D. A. Loy, K. J. Shea, Chem. Rev. 1995, 95, 1431-1442). If basic polymers constitute the polymer matrix, acid-catalyzed sol-gel condensation is unsuitable as a method, since the salt formation occurring due to acid-base interactions makes the polymer insoluble in organic solvents. For a polybenzimidazole matrix, the incorporation of organically modified Na+ bentonite is known in the literature, the resulting bentonite-PBI nanocomposite having elevated thermal stability compared with unmodified PBI (T. Seckin, Materials Science and Engineering B 107 (2004) 166-171). It is known that phosphotungstic acid immobilized on silica can be incorporated as a filler in a polybenzimidazole matrix for use as a polymer electrolyte in fuel cells (P. Staiti, M. Minutoli, S. Hocevar, J. Power Sources 90 (2000) 231-235). Such membranes are produced by addition of a mixture of silica and phosphotungstic acid to a solution of polybenzimidazole in N,N-dimethylacetamide. They have low proton conductivity above 110° C. For AB-PBI as a polybenzimidazole modification, an improvement of conductivity in the condition doped with phosphoric acid was achieved by direct incorporation of phosphomolybdic acid (P. Gomez-Romereo, J. A. Asensio, S. Borros, Electrochimica Acta 50 (2005), 4715-4720). In this case, the heteropoly acid was dissolved in a solution of AB-PBI in methanesulfonic acid before the membrane production process, and homogeneous casting solutions were obtained.